Method and apparatus for magnetic force control of a scanning probe

ABSTRACT

A method and apparatus of magnetic force control for a scanning probe, wherein a first magnetic source having a magnetic moment is provided on the scanning probe and a second magnetic source is disposed external to the scanning probe to apply a magnetic field in a direction other than parallel, and preferably perpendicular, to the orientation of the magnetic moment, from the second magnetic source to the first magnetic source to produce a torque related to the amplitude of the applied magnetic field acting on the probe. By controlling the amplitude of the applied magnetic field, the deflection of the scanning probe is maintained constant during scanning by the scanning probe. An output signal related to the amplitude of the magnetic field applied by the second magnetic source is produced and is indicative of a surface force applied to the probe. The invention can also be used to apply large forces during scanning for applications such as nanolithography or elasticity mapping.

This application is a continuation of application Ser. No. 08/290,091filed Aug. 15, 1994 which application is now U.S. Pat. No. 5,670,712.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to scanning probe microscopy and, has applicationto specific types of scanning probe microscopes, such as the atomicforce microscope. More particularly, the present invention relates to amethod and apparatus of magnetic force control for scanning probemicroscopy. Also, the invention has broader applications, such as in thefield of nanolithography.

2. Discussion of Background

Atomic force microscopes are typically devices which have a relativelysharp tip and use low forces to profile a sample surface down to atomicdimensions. Often, atomic force microscopes have a tip on a flexiblelever with the vertical position of the tip of the probe detected by adetector. Various detectors have been utilized, such as tunneling tips,optical interferometers and optical beam deflection. Also, variouscapacitive and inductive proximity detectors are known.

In operation, the atomic force microscope typically will scan the tip ofthe probe over the sample while keeping the force of the tip on thesurface constant, such as by moving either the base of the lever or thesample upward or downward to maintain deflection of the lever portion ofthe probe constant. Therefore, the topography of a sample may beobtained from data on such vertical motion to constructthree-dimensional images of the surface topography. It is also knownthat atomic force microscopes utilize analog and digital feedbackcircuits to vary the height of the tip of the probe or the sample basedupon the deflection of the lever portion of the probe as an input. As toatomic force microscopes, see also for example, U.S. Pat. No. 5,025,658and 5,224,376.

The contrast in an atomic force microscope image is due to spatialvariations in the force between a probe and the surface. The most commonmode of operation uses the short-ranged, Pauli exclusion ("hard-wall")force that arises when the electron clouds of atoms in the tip overlapthose in the sample. It is the extremely high gradients of this forcewhich give rise to the excellent resolution of the microscope. However,there are at least three disadvantages to operating in this mode. Thedownward forces on the entire tip (e.g. van der Waals, capillary forcesand tracking forces) must be opposed by upward forces on the few atomsin contact with the surface. The large stresses generated can deform thetip and sample surface thus reducing lateral resolution. Anotherdisadvantage is that when the tip slides in contact with the sample,there are additional lateral (frictional) forces that sometimes cannotbe distinguished from normal forces. These lateral forces may also causedamage to the tip and sample. Finally, since the distance dependence ofthe "hard-wall" force is too steep to be resolved, the contrast isgenerated almost entirely by topography and, therefore, chemical ormaterial information is unavailable.

Non-contact imaging has been used to circumvent these disadvantages. Theforces that generate contrast in this mode are usually attractive (forexample van der Waals, electrostatic, and magnetic). If the gradients inthese forces exceed the stiffness of the cantilever, a mechanicalinstability will occur and the tip will snap to the surface. Thus, thereis a range of tip-sample separations that are mechanically unstable and,therefore, unsuitable for imaging. For this reason, non-contact imagingis usually performed at large (>10 nm) tip-sample separations. However,resolution increases with decreasing tip-sample separation, so imagingat a small tip-sample separation would be desirable if stability couldbe achieved. Stability can be achieved by the application of anexternally controllable force on the tip and the use of feedback.Externally controllable forces have been applied in atomic forcemicroscopes operating in air using electrostatic forces (S. A. Joyce andJ. E. Houston, Rev. Sci. Instr., 62, 710 (1991); G. I. Miller, J. E.Griffith and F. R. Wagner, Rev. Sci., Instrum., 62, 705 (1991); and D.A. Grigg, P. E. Russell and J. E. Griffith, Ultramicroscopy, 42-44, 1504(1992)), thermal stresses (J. Mertz, O. Marti and J. Mlynek, Appl. Phys.Lett., 62, 2344 (1993)), magnetic force gradients (B. Gauthier-Manuel,Europhys. Lett., 17, 195 (1992)) and inertial forces (P. J. Bryant, H.S. Kim, R. H. Deeken and Y. C. Cheng, J. Vac. Sci. Technol., A 8, 3502(1990)).

The importance of minimizing imaging forces to decrease sample damageand to improve resolution in contact-mode atomic force microscopy iswell known. In non-contact atomic force microscopy better forceresolution also means increased spatial resolution. Since the forceresolution is set by the smallest deflection of the cantileverdetectable, weaker cantilevers typically mean lower imaging forces andare therefore desirable. However, on the other hand, there are manyapplications where large forces or spring stiffness are advantages orperhaps even necessary. Although larger forces and spring constants canbe achieved with stiffer cantilevers, this almost always leads todecreased force resolution and higher imaging forces. It is known that acantilever may experience a mechanical instability and may snap to thesurface when the attractive force gradient exceeds the spring constant.Although it is known to use stiffer cantilevers to allow surface forcesto be measured at closer surface separations, such use of stiffercantilevers also decreases the force resolution.

Another apparatus known in the prior art is the surface force apparatuswhich measures the force between two surfaces. However, with the surfaceforce apparatus, the length scale is much larger than with the scanningprobe microscope. Typically, for example, the radii of curvature of thesurfaces are 2 cm which may limit the choice of samples. Further, withthe surface force apparatus, the apparatus does not have imagingcapabilities.

Also, it is known to use magnetic field gradients to generate magneticforces to act on a magnetic moment rather than torques on a moment. See,for example, "Use of magnetic forces to control distance in surfaceforce apparatus", Stewart et al, Meas. Sci. Techno. 1, pages 1301-1303(1990); and "Direct Measurement of the Short-Range Interaction between aTungsten Tip and a Mica Surface", B. Gauthier-Manuel, EurophysicsLetters, 17, pages 195-200, (1992, published in December, 1991).

FIG. 6A is a schematic illustration of the conventional application of amagnetic field gradient to produce a force F acting on a magnetic momentm. In FIG. 6A, a magnetic moment m is oriented parallel to the directionof the magnetic field gradient. While a force is experienced that isproportional to the field gradient, there is no force in a constantfield.

SUMMARY OF THE INVENTION

Accordingly, it is the object of the present invention to provide anovel method and apparatus utilizing magnetic force control for scanningprobe microscopy to apply large forces in a contact-mode in applicationssuch as nanolithography or elasticity mapping, and when used in afeedback loop in a non-contact mode, to provide a large "electronicspring constant", and to minimize loss of sensitivity.

Another object of the present invention is to permit feedback to be usedto hold the cantilever portion of the probe at a constant deflectionthroughout the force measurement, permitting the distance between thetip portion of the probe and the sample to be controlled, such as bypiezoelectric control.

A further object of the method and apparatus of the present invention isto enable measurement of attractive surface forces down to contact.

Another object of the present invention is to improve non-contactimaging of van der Waals forces at small separations, such as on theorder of less than 1 nm, where the force gradient exceeds the cantileverspring constant.

These and other objects are achieved according to the present inventionby providing a new improved method and apparatus for magnetic forcecontrol for a scanning probe, the method including disposing of a firstmagnetic source having a magnetic moment on the scanning probe,disposing a second magnetic source external to the scanning probe toapply a magnetic field in a direction other than parallel to theorientation of the magnetic moment from the second magnetic source tothe first magnetic source so as to produce a torque acting on thescanning probe related to the amplitude of the magnetic field applied.

Preferably, the magnetic torque is applied by applying a uniformmagnetic field perpendicular to the orientation of the magnetic moment.The method of magnetic force control according to the present inventionpreferably also includes controlling the amplitude of the magnetic fieldto maintain the deflection of the scanning probe constant duringscanning by the scanning probe.

The method of magnetic force control according to the present inventionmay also include sensing the deflection of the scanning probe as causedby a surface force and applying the magnetic field with an amplitudederived based on the sensed deflection to maintain the deflection of thescanning probe constant. In the method of magnetic force controlaccording to the present invention, it is preferable to apply at leastone of a proportional gain and an integral gain to a signal related tothe sensed deflection to derive the amplitude of the magnetic field.

In the method of magnetic force control according to the presentinvention, it is preferable that the scanning probe include a rockingbeam balance having a pivot portion and a cantilever portion extendingfrom the pivot portion, and wherein disposing the first magnetic sourceincludes disposing the first magnetic source on the cantilever portionof the rocking beam balance.

In the method of magnetic force control according to the presentinvention, the scanning probe may also alternatively include acantilever portion having a fixed end and a free end, and whereindisposing the first magnetic source includes disposing the firstmagnetic source on the cantilever portion. Disposing the first magneticsource may include disposing a magnetic film having a permanent magneticmoment lying in a direction that is along the cantilever portion.

In the method of magnetic force control according to the presentinvention, disposing the first magnetic source may include magnetizingthe magnetic film after depositing the magnetic film on the cantileverportion to orient the magnetic moment.

The method of magnetic force control according to the present inventionadditionally may include patterning a magnetic film as the firstmagnetic source to form a structure lying along the cantilever portionand constraining the magnetic moment of the magnetic film to lie in adirection that is along the structure.

The method of magnetic force control according to the present inventionmay include disposing a magnet as the first magnetic source on the freeend of the cantilever portion, and disposing the magnet to have amagnetic moment that lies in a direction that is along the cantileverportion.

The magnetic force control apparatus for a scanning probe according tothe present invention includes a probe provided with a first magneticsource having a magnetic moment, and a second magnetic source forapplying an external magnetic field in a direction other than parallelto the orientation of the magnetic moment so as to produce a torqueacting on the probe related to the amplitude of the magnetic fieldapplied.

The magnetic force control apparatus according to the present inventionmay preferably include control means for controlling the amplitude ofthe magnetic field applied by the second magnetic source to maintaindeflection of the probe constant. The control means may include meansfor sensing the deflection of the probe as caused by a surface force andmeans for controlling the amplitude of the magnetic field applied to thefirst magnetic source based on the sensed deflection of the probe. Themeans for controlling the amplitude may include at least one of aproportional gain and an integral gain in processing a signal related tothe sensed deflection of the probe.

In an embodiment of the magnetic force control apparatus of the presentinvention, the second magnetic source includes an electromagnet and thefirst magnetic source includes a magnet positioned on the probe.

In another embodiment of the magnetic force control apparatus of thepresent invention, the second magnetic source includes an electromagnetand the first magnetic source includes a thin magnetic film on theprobe.

In the magnetic force control apparatus according to the presentinvention, the probe preferably may include a rocking beam balancehaving a pivot portion and a cantilever portion extending from the pivotportion and on which the first magnetic source is disposed.

In another embodiment of the magnetic force control apparatus accordingto the present invention, the probe may alternatively include acantilever portion having a fixed end and a free end, and wherein thefirst magnetic source is provided on the cantilever portion. The firstmagnetic source may include a magnetic film having a magnetic momentthat preferably lies in a direction along the cantilever portion.

In another embodiment of the magnetic force control apparatus of thepresent invention, the first magnetic source includes a magnet disposedon the free end of the cantilever portion, and the magnet has a magneticmoment that lies in a direction that is along the cantilever portion.

In the magnetic force control apparatus according to the presentinvention, it is preferable that the magnetic film as the first magneticsource include a material that is a metallic alloy, such as acobalt-chromium alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of an embodiment of an apparatususing magnetic force control of the present invention.

FIG. 2 is a schematic block diagram of an embodiment of a magnetic forcecontrol apparatus of the present invention.

FIGS. 3A and 3B are schematic illustrations of an embodiment of arocking beam balance for a magnetic force control apparatus of thepresent invention.

FIG. 3C is a schematic illustration of an embodiment of an apparatususing magnetic force control of the present invention having a rockingbeam balance.

FIG. 3D is a schematic illustration of an embodiment of an apparatususing magnetic force control of the present invention including a probehaving a cantilever portion with a fixed end and a free end and having amagnetic film as the first magnetic source.

FIG. 3E is a schematic illustration of an embodiment of an apparatususing magnetic force control of the present invention including a probehaving a cantilever portion with a fixed end and a free end and having amagnet as the first magnetic source.

FIG. 3F is a schematic illustration of an embodiment of an apparatususing magnetic force control of the present invention similar to theapparatus illustrated in FIG. 3D, except the second magnetic source isillustrated as including two electromagnetic coils disposed in opposingrelation to each other with the cantilever portion of the probe locatedbetween the two coils.

FIGS. 4A-4C are graphs illustrating results obtained by the conventionalatomic force microscope method and apparatus and by that of the presentinvention. FIG. 4A is a conventional atomic force microscope force curvedisplaying cantilever deflection versus sample displacement and in whichno magnetic feedback is used. FIGS. 4B and 4C illustrate performanceutilizing magnetic force feedback according to the present invention,with FIG. 4B illustrating cantilever deflection and force versus sampledisplacement, and FIG. 4B illustrating force versus sample displacement.

FIG. 5A is a schematic illustration of a known application of a forcewith a magnetic field gradient.

FIG. 5B is a schematic illustration of generating a force by a magnetictorque generated by application of a magnetic field as in the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, and moreparticularly to FIGS. 1 and 2 thereof, a sample 2 having a top surface 4is illustrated. The sample 2 is placed on a sample holder 6. A scanningprobe, probe 8, has a tip portion 10 and a cantilever portion 12. Afirst magnetic source 14 is positioned on the probe 8 in the area of thetip portion 10. The first magnetic source has a magnetic moment m. Asecond magnetic source 16 having a pole face 18 is positioned externalto the probe 8, preferably above the first magnetic source 14, andapplies a torque by a magnetic field B to the first magnetic source 14so as to produce a force on the tip 10 as illustrated in FIG. 2.

The magnetic field B is applied in a direction other than parallel tothe orientation of the magnetic moment m of the first magnetic source 14to produce a magnetic torque acting on the magnetic moment m of thefirst magnetic source 14 to thereby produce a torque acting on thecantilever portion 12 of the probe 8 related to the amplitude of themagnetic field B applied. It is preferable that the magnetic torque begenerated by means of a uniform magnetic field applied in a directionperpendicular to the orientation of the magnetic moment m. It ispreferable that the orientation of the magnetic moment m be parallel tothe cantilever portion 12 of the probe 8. Referring to FIG. 5B, forexample, a force F generated by application of a magnetic field B actingon a magnetic moment m to apply a magnetic torque is illustrated. Thetorque on the magnetic moment m or dipole D in FIG. 5B results in aforce at the end of the magnet of the order mB_(z) / where is the lengthof the magnet. The force is proportional to the magnetic field B. Theforce on the same magnet in a magnetic field gradient applied parallelto tee magnetic moment would be ##EQU1## where ##EQU2## is the fieldgradient in the direction along the magnetic moment. Comparing theexpressions for the force generated using a magnetic torque versus amagnetic field gradient, it is seen that the quantity B_(z) / plays therole of an effective field gradient. That is, to compare the performanceof the two methods, the quantities ##EQU3## and B_(z) / can be directlycompared.

Preferably, the first magnetic source 14 includes a small rare earthmagnet in a size range of from 10 μm to 100 μm, one example of such sizebeing less than 50 microns, preferably on the back side 11 of thecantilever portion 12, which is affixed thereon by means of epoxy, forexample. The magnet may also be affixed to the front side 13, ormagnet(s) may be affixed to both the front side 13 and back side 11 ofthe cantilever portion 12. FIGS. 1, 2, and 3E illustrate the firstmagnetic source 14 as including a magnet, and as illustrated in theseFIGS., the magnetic moment m of the magnet preferably lies in adirection that is along the cantilever portion 12 of the probe 8.

Referring to FIGS. 3A, 3B, 3C, 3D, and 3F, the first magnetic source 14may include a thin magnetic film T which is deposited on the cantileverportion 12 of the probe 8. The magnetic film T is preferably depositedon the back side 11, and may also be deposited on the front side 13, oron both the back side 11 and front side 13 of the cantilever portion 12.The magnetic film T on the cantilever portion 12 may also be used inconjunction with a discrete magnet on the cantilever portion 12 so as totogether constitute the first magnetic source 14. For example, adiscrete magnet could be positioned on the top of the cantilever portion12, and a thin magnetic film T on the bottom of the cantilever portion12. The thin magnetic film T preferably has a thickness in the range offrom 10 nm to 100 nm. Also, the magnetic film T is preferably a metallicalloy, such as a chromium-cobalt alloy. The magnetic film may includeiron, nickel, and/or cobalt.

While a magnet that is glued to the cantilever portion 12 of probe 8,such as by epoxy, may possibly be more preferable than a magnetic thinfilm T evaporated or deposited on the cantilever portion 12 of the probe8, for better reproducibility and mass production, it is desirable tomicrofabricate the magnet, as the first magnetic source 14, on the backside 11 of the cantilever portion 12 of the probe 8.

There are many possibilities as to ways to microfabricate the firstmagnetic source 14 on the cantilever portion 12 of the probe 8 usingthin magnetic films. In general, a structure with as large a magneticmoment along the length of the cantilever as possible is desired. Tothis end, the thin film T could be patterned (for example, usingphotolithographic techniques) as illustrated in FIGS. 3B and 3D, to forma structure ST lying along the cantilever portion 12 of the probe 8having a long narrow direction along the cantilever portion 12 so thatshape anisotropy will constrain the magnetic moment m of the magneticfilm T to preferably lie along the cantilever portion 12 in a directionthat is along the structure ST. The choice of the magnetic material isalso important. Alloys and deposition conditions can be chosen to causethe magnetic moment m to preferably lie in the plane of the magneticfilm T, desirably in a direction that is along the cantilever portion 12of the probe 8. There are many variations of alloys of Cobalt andChromium with just these properties that have been designed for magneticrecording applications. Referring to FIGS. 3B and 3D, the thin film Tcould also be magnetized after deposition of the thin film T to causethe magnetic moment m to preferably lie in a direction that is along thecantilever portion 12 and to provide the magnetic poles N and S themagnetic moment m. Note that the magnetic moment m could also lieanti-parallel to the direction shown in FIGS. 3B, 3D, and 3F.

Referring to FIG. 3D, in the case of a thin film T lying on thecantilever portion 12, the interpretation for how the force is generateddiffers slightly because one of the poles S of the magnetic moment m isfixed on the substrate at the fixed end 17 and cannot move under theapplication of a magnetic field. Then the other pole N behaves like amonopole at the free end 19 of the cantilever portion 12 in that it doesfeel a force in a constant magnetic field.

It is further preferable that the probe 8 include a microfabricatedrocking beam balance. The rocking beam balance has the advantage that itcan be made weak torsionally so as to have a small spring constant but,on the other hand, the magnet is preferably rigid to provide a moreefficient lever arm for the magnetic torque. An example of a rockingbeam balance is described in "Rocking beam electrostatic balance for themeasurement of small forces" by Miller et al, Rev. Sci. Instrum. 62(3),pages 705-709 (March, 1991).

An example of a rocking beam balance for use in the present inventionschematically is illustrated in FIGS. 3A-3C. Referring to FIGS. 3A, 3Cand 3B, the rocking beam balance has a probe 8. The probe 8 has a beamportion 12, a pivot portion 15 supporting the beam portion 12, with thebeam portion 12 extending from the pivot portion 15, and a tip portion10. The probe 8, including the beam portion 12 and pivot portion 15, maybe microfabricated from silicon or silicon nitride. See, for example,"Microfabrication of Cantilever Styli for the Atomic Force Microscope",T. R. Aubrecht, S. Akamine, T. E. Carver, and C. F. Quate, J. Vac. Sci.Technol. A8(4) pg. 3386-3396 (July/August 1990). The first magneticsource 14 includes a thin magnetic film or magnetic coating T that ispreferably formed on the back side 11 of beam portion 12. The beamportion 12 supported by the pivot portion 15 is adapted for rocking orpivotal movement about the pivot portion 15 with a portion of the beamportion 12 being able to move in a space Sp formed in the probe 8. Themagnetic moment m of the first magnetic source 14 is preferably orientedparallel or anti-parallel to the arrow in FIGS. 3A and 3B.

FIG. 3C schematically illustrates the rocking beam balance of FIGS. 3Aand 3B for use in magnetic force control according to the presentinvention. In FIG. 3C, the tip portion 10 of the probe 8 is positionedover the sample surface 4 of sample 2 for scanning the surface 4. Thesecond magnetic source 16 is positioned external to the probe 8,preferably over the beam portion 12, to apply a magnetic field B to themagnetic moment m of the first magnetic source 14.

Also, the probe 8 may include a standard atomic force microscopecantilever modified to include the first magnetic source 14, suchstandard atomic force microscope cantilever being available from DigitalInstruments, Santa Barbara, Calif., (such as a silicon nitridecantilever, part number NP or NP-S, or a silicon cantilever, part numberESP).

Another example of a probe 8 for use in the present invention isschematically illustrated in FIGS. 3D, 3E, and 3F. Referring to FIGS.3D-3F, the probe 8 has a cantilever portion 12 having a back side 11 anda front side 13, and a tip portion 10. The cantilever portion 12 has afixed end 17 and a free end 19. The fixed end 17 is fixed in a substrateportion 21 of the probe 8. The probe 8, including the cantilever portion12, may be microfabricated from silicon or silicon nitride similar tothe fabrication of the probe 8 including the cantilever portion 12illustrated in FIGS. 3A-3C.

Referring to FIGS. 3D and 3F, the first magnetic source 14 includes athin magnetic film or magnetic coating T that is preferably formed onthe back side 11 of the cantilever portion 12. The materials andformation of the magnetic film T are similar to those previouslydiscussed herein, the magnetic film T preferably being a magnetic alloy,such as a cobalt-chromium alloy. The magnetic film T has one magneticpole S at the fixed end 17 and another magnetic pole N at the free end19 of the cantilever portion 12. In the embodiment illustrated in FIGS.3D-3F, the fixed end 17 does not move under the application of amagnetic field while the free end 19 is adapted for movement under theapplication of a magnetic field. A magnetic moment m of the firstmagnetic source 14 is preferably oriented parallel or anti-parallel tothe arrow in FIGS. 3D and 3F, and the magnetic moment m would preferablylie in a direction that is along the cantilever portion 12 of the probe8.

FIG. 3E is similar to FIG. 3D with the difference that the firstmagnetic source 14 in FIG. 3E includes a magnet, such as a rare earthmagnet, positioned on the free end 19 of the cantilever portion 12.Similar to FIG. 3D, the free end 19 in FIG. 3E is adapted to move underthe application of a magnetic field while the fixed end 17 does not moveunder the application of a magnetic field.

FIGS. 3D-3F schematically illustrate a probe 8 having a cantileverportion 12 with a fixed end 17 and a free end 19 for use in magneticforce control according to the present invention. In FIGS. 3D-3F, thetip portion 10 of the probe 8 is positioned over the sample surface 4 ofsample 2 for scanning the surface 4. The second magnetic source 16 ispositioned external to the probe 8, preferably over the cantileverportion 12 as illustrated in FIGS. 3D and 3E, to apply a magnetic fieldB to a magnetic moment m of the first magnetic source 14. In FIG. 3F,the second magnetic source 16 includes electromagnetic coils 16a and 16bpositioned external to the probe 8 with the cantilever portion 12 of theprobe 8 positioned between the electromagnetic coils 16a and 16b toapply the magnetic field B to a magnetic moment m of the first magneticsource 14.

It is preferable that the magnetic moment m of the first magnetic source14 be oriented along the length of the cantilever portion 12 asillustrated by the arrow in FIGS. 1, 2, and 3A-3F. In one embodimentimplemented according to the invention, the second magnetic source 16includes an electromagnet having a pole face 18 in the size range offrom 1 mm to 2 cm, positioned in a range of from 100 μm to 2 cm, fromand preferably above the cantilever portion 12 of probe 8, one examplebeing a pole face 18 of 5 mm positioned 1 cm above the cantileverportion 12, to provide a magnetic field perpendicular to the orientationof the magnetic moment m, the magnetic field being fairly uniform on thelength scale of the first magnetic source 14, such as the magnet or thethin magnetic film T, on the cantilever portion 12.

Referring to FIGS. 1, 2, and 3C-3F, a means for measuring the deflectionof the probe 8 during the measurement of a surface force is illustrated.For example, a light source 20, such as a laser diode, emits a lightbeam L. The light beam L is reflected off the cantilever portion 12 ofthe probe 8 and the reflected beam R is detected by a detector 22, suchas a segmented photodiode.

Controller 24, as illustrated, functions to control relative movementbetween the probe 8 and sample 2 during scanning in a conventionalmanner such as in scanning probe microscopy, for example. Controller 24also functions in the control of the application of the magnetic field Bfrom the second magnetic source 16 to the first magnetic source 14 inthe present invention. The controller 24 illustrated in FIGS. 1, 2, and3C-3F, typically is a computer controlling various functions duringmeasurement of a surface force or during movement of the probe 8 on orover a surface. The controller 24 also receives and processes datareceived during such measurement of a surface force or during suchmovement of the probe 8, such as data on the measurement of or sensingof the deflection of the probe 8. The controller 24 is typically acomputer that includes the requisite logic and circuitry to accomplishsuch control, data storage, and data processing.

In operation, for example, the controller 24 typically provides a signalto the light source 20 to generate the light beam L to be reflected offthe surface of the cantilever portion 12 of the probe 8. A signalrelated to the reflected beam R sensed by the detector 22 is provided tothe controller 24. The controller 24, typically, includes ananalog-to-digital converter, for example, for converting an analogsignal from the detector 22 into a digital signal; and a programmeddigital computer for receiving the digital signal from theanalog-to-digital converter and calculating an appropriate output oroutputs, as disclosed, for example, in U.S. Reissue Pat. No. RE 34,331.The controller 24 executes proportional gain, integral gain ordifferential gain processing steps, or a combination thereof, to producean output signal related to the sensed deflection from the detector 22,to derive an amplitude of the magnetic field B to be applied.

Each form of gain (proportional, integral, differential) produces anoutput signal related to the sensed deflection to keep the cantilever ata desired deflection. More accurately, the gains produce an outputsignal related to the error signal. The error signal is the differencebetween the deflection signal and some desired deflection (known as thesetpoint) and is a measure of how far away the cantilever is from thedesired deflection. Proportional gain produces a signal directlyproportional to the error signal, while integral gain produces a signalproportional to the integral with respect to time of the error signal,and differential gain produces a signal proportional to the timederivative of the error signal. Two or all three of these gain signalscan be added together to produce an output signal to be supplied to theelectromagnet. It has been found that integral gain with a small amountof proportional gain is particularly useful.

The controller 24, typically, controls a power source 26, such as acurrent source, to control the current supplied to the second magneticsource 16 which then applies the magnetic field B to the first magneticsource 14; this results in a torque on the first magnetic source 14, andin turn a force F at the tip 10. The power source 26 may be combinedwith the controller 24 as schematically illustrated in FIGS. 1 and3C-3F. The controller 24 controls the power source 26 to supply currentof a magnitude so that the amplitude of the magnetic field B and,therefore, the resultant force F, is applied in relation to the surfaceforce acting on the probe 8, such as in a proportional relation theretowhen using proportional gain processing, for example. Therefore, thecurrent supplied, the amplitude of the magnetic field B applied, and theresultant force F applied may vary with time, depending on the surfaceforce acting on the probe 8. It is further preferable that the magneticfield B generated by the second magnetic source 16 be a uniform magneticfield oriented perpendicular to the orientation of the magnetic moment mof the first magnetic source 14. Such a field could also be produced byplacing the cantilever at the center of a pair of electromagnetic coils16a, 16b illustrated in FIG. 3F, such as a pair of Helmholtz coils, forexample.

Therefore, with the method and apparatus of the present invention, amagnetic field B is applied by the second magnetic source 16 to act onthe magnetic moment m of the first magnetic source 14 to maintaindeflection of the cantilever portion 12 of the probe 8 constant, such asduring non-contact imaging as the probe 8 and the sample 2 are movedrelative to each other or, in particular, during the measurement ofsurface forces, as in FIG. 4. The size of the first magnetic source, theamplitude of the magnetic field B, and the amplitude of the force Fgenerated depend upon the particular use and application. However,typically forces on the order of 2 to 500 nN can be generated with anelectromagnet and a small magnet on the cantilever portion of the probein atomic force microscope applications of the invention.

The strength of the field generated second magnetic source typicallywill be in the range of 0.01 T to 0.1 T, and the strength of themagnetic moment of the first magnetic source will typically be in therange of 10⁻⁴ EMU to 10⁻⁶ EMU. For example, in atomic force microscopyapplications, the magnetic field B generated may be 0. 01 T(100 G) 2 mmfrom the pole face of the electromagnet.

When a current is supplied from the power source 26, such as a currentsource, to the second magnetic source 16, such as an electromagnet, themagnetic moment of the first magnetic source 14 experiences a magnetictorque that tends to align it with the field. A reversal of theelectromagnetic current reverses the field and therefore the torque. Ifthe tip portion 10 of the probe 8 is not in contact with the surface 4of the sample 2, such as during force measurements or non-contactimaging, and a surface force is acting on the tip portion 10, a torqueis applied on the cantilever portion 12 of the probe 8. This torque canbe cancelled magnetically using a feedback loop to detect the cantileverdeflection. This is schematically indicated in FIGS. 1, 2 and 3C-3F bythe reflected beam R, detected by the detector 22 providing a signal tothe controller 24 which controls the power source 26 to supply theappropriate current to the second magnetic source 16 to apply themagnetic field B to the cantilever portion 12 of the probe 8 to producea torque to magnetically cancel this torque. The amplitude of themagnetic field B therefore can vary. Therefore, the application of themagnetic field B and the resultant force F so applied maintains thedeflection of the cantilever portion 12 of the probe 8 constant, and thecurrent supplied to the second magnetic source 16 to keep it so isproportional to the surface force acting on the probe 8 at the tipportion 10. Thus, with the present invention, the current to the secondmagnetic source 16 required to maintain the deflection of the probeconstant is a direct measure of the surface force acting on the probetip portion 10.

The force sensitivity may be determined by the spring constant of thecantilever portion 12 of the probe 8 and the sensitivity of the positiondetector, such as detector 22. However, in the method and apparatus ofthe present invention, the cantilever may act with an effective"electronic spring constant" which can be significantly larger than thatof the cantilever portion 12 of the probe 8. If the tip portion 10 ofthe probe 8 is in contact with the surface 4 of the sample 2, themagnetic torque can supply a force on the tip portion 10 through a leverarm along the cantilever portion 12, and may therefore be used togenerate large forces on the sample 2 for nonolithography or elasticitymeasurements, for example.

FIGS. 4B and 4C illustrate the performance of the magnetic force controlof the present invention in comparison with the conventional controlshown in FIG. 5A. These figures illustrate the results of forcemeasurement between a silicon nitride tip and a piece of silicon nitridewafer in water (>18MΩ) with and without magnetic force feedback.

All measurements were performed using a Nanoscope II AFM (atomic forcemicroscope), supplied by Digital Instruments, Inc., utilizing opticallever detection with a standard fluid cell. Triangular silicon nitridecantilevers 200 μm in length (the altitude as measured from the baseportion of the triangular cantilever) with 36 μm wide legs were modifiedby attaching a small permanent magnet to the backside of the cantilever.The magnets were made by grinding a large SmCo magnet and collectingmagnetic shards from the particle stream using a permanent magnet behinda glass coverslip. The magnets were then removed from the coverslipusing glass micropipettes and a three-way micropositioner. A separatemicropipette and positioner were used to place a small drop of epoxy onthe backside of the cantilever. After allowing the epoxy to cure for tento fifteen minutes, the magnet could be transferred from themicropipette to the free end on the cantilever. This entire operationwas performed in the presence of a magnetic field so that theorientation of the magnet could be controlled. The moment of the magnetwas oriented along the length of the cantilever.

The spring constant of the cantilever was estimated by measuring theresonant frequency of the cantilever before the magnet was attached. Thecantilevers used were all from one location in a single wafer and hadspring constants of 40±10 pN/nm. The cantilever could then be used as ananogram balance and measurement of the resonant frequency afterattaching the magnet gave a value for the mass of the magnet and epoxy.This was typically 30 to 200 ng. corresponding to a cube with sides 16to 30 μm. The magnets were fairly thin (e.g., 40×40×10 μm) and werepositioned flat on the cantilever.

A solenoid with a magnetic core was used to apply a force on thecantilever. The core was a piece of Permendur (a high permeability, lowcoercivity alloy) with rectangular cross-section about 3 by 4 by 25 mmwith a ground pole tip to focus the field and provide optical clearance.About 200 turns of #30 copper magnet wire were wound on the core givingthe electromagnet a resistance of 4 Ω and an inductance of 500 μH at 1kHz. The coil was driven by a power amplifier current limited to 1 A. A100 Ω power resistor was added in series to the coil to increase thefrequency response of the coil. The electromagnet was mounted on a threeway micropositioner so that it could be positioned accurately above thecantilever without blocking the laser beam used for position detection.

Balance between the tip-sample force and magnetic force was achieved byfeeding the cantilever deflection signal into a feedback amplifier whichdrove the electromagnet through a current amplifier. Although bothproportional and integral gain were used, integral gain was typicallymore important. Any changes in the force applied to the tip by thesample were balanced by the magnetic torque which maintained acantilever deflection corresponding to the set point of the feedbackamplifier.

The voltage to the z piezo was produced using a function generator and ahigh voltage amplifier. Both the output from the feedback amplifier andthe deflection signal from the microscope could be displayed against thez voltage on a storage oscilloscope. The data was digitized andtransferred to a computer for display.

The force generated by the electromagnet was calculated by recording thecantilever deflection for a given current and then multiplying by thecantilever spring constant. The cantilever deflection calibration (V/nm)was measured from the slope of the constant compliance (contact region)of a force curve on mica. Depending on the size and properties of thepermanent magnet, maximum forces from 2 to 500 nN could be applied tothe tip of the cantilever.

FIG. 4A is a standard atomic force microscope force curve displayingcantilever deflection versus sample position or displacement withoutmagnetic force feedback measured between a silicon nitride tip and asilicon nitride wafer in water in the presence of a contamination layer.Magnetic force control according to the present invention was notutilized in obtaining the data in FIG. 4A. Referring to the curveillustrated in FIG. 4A, there was noted a very slight repulsion(approximately 100 pN) and then an instability occurred and thecantilever snapped to the surface from about 60 nm at point 1, wherewetting forces caused cantilever instability at this tip-sampleseparation. When the load was increased with the tip in contact with thesurface, a large repulsive force due to contact with the surface wasexperienced from 60 nm to 0 nm. Upon decreasing the load, the cantileveradhered to the surface and then pulled away from the surface about 20 nmbefore experiencing another instability at point 2. The cantileverjumped from point 2 to a position about 65 nm above the surface at point3 where it slowed, and then finally jumped to the zero force line.

The force curve in FIG. 4A illustrates a wetting force due to acontamination layer on the sample. Any calculation of attractive van derWaals forces between the tip and the sample predicts cantileverinstabilities at distances less than 5 nm even for big tips (R=100 nm),large Hamaker constants (A=10⁻¹⁹ J), and weak cantilevers (k=50 pN/nm).The slight repulsion noted before the instability also evidenced thepresence of a contamination layer. Interfaces between two liquids areoften charged, so double-layer forces are expected. Finally, the factthat the cantilever did not jump cleanly from the surface at point 2 andthen slowed at point 3 in FIG. 4A is consistent with a wetting force.Wetting of the sides of the tip as it withdraws from the surface couldaccount for the flat lift-off at point 2 and a meniscus drawn into thewater could account for the slowing of the lever at point 3.

FIGS. 4B and 4C show the performance of magnetic force control of thepresent invention utilizing magnetic force feedback for forcemeasurement between a silicon nitride tip and a silicon nitride wafer inwater in the presence of a contamination layer. FIG. 4B shows thedeflection signal recorded while the feedback was running (the errorsignal). The cantilever deflection was maintained constant to within 2nm (80 pN) up to contact. Therefore, this evidenced that the feedbackwas balancing the forces applied to the cantilever. FIG. 4C is relatedto the current supplied to the electromagnet to maintain the constantcantilever deflection. Data is only shown up to the point of contact.The feedback did not maintain constant cantilever deflection long afterthe tip came into contact with the surface because doing so would haveresulted in applying relatively large forces likely to deform the tipand the surface. Therefore, the power amplifier was current limited to avalue corresponding to a few nanoNewtons of repulsive force but was notcurrent limited in the attractive force direction.

Since the cantilever was held at a constant separation throughout themeasurement, FIG. 4C can be interpreted as a true force versustip-sample separation curve. Because the cantilever deflection wasmaintained constant throughout the measurement, the horizontal axis canalso be read as tip-sample separation. The data in FIG. 4C shows thesudden onset of an attractive force at about the same tip-sampleseparation at which the instability occurred in FIG. 4A. A lower limitof about 3 nN/nm can be placed on the maximum attractive force gradient.Since the cantilever used had a spring constant about 60 times smaller,this is consistent with the fact that an instability was observed inFIG. 4A.

FIGS. 4A-4C demonstrate the use of magnetic force control of the presentinvention to measure a wetting force in a regime that is mechanicallyunstable without feedback. Thin films commonly absorb to surfaces inboth liquid and air, and can produce forces when in contact with thetip. These types of layers are often present and can make imagingdifficult or impossible because they apply an uncontrollable loadingforce to the tip which can be much larger than the smallest trackingforces that can be applied (<10 pN). Magnetic force control of thepresent invention using force feedback can be used to offset thesedetrimental forces during imaging. Since ferromagnetic materials aremuch less common than dielectric materials, use of magnetic forcesextends the applicability of feedback control. Many materials used inatomic force microscope construction are not ferromagnetic (e.g. piezos,glass, aluminum, and stainless steel) so they can come between thecantilever and magnetic field source without affecting the field. Also,most liquids have a magnetic permeability of essentially unity and aretherefore invisible to magnetic fields. Thus, the presence of liquidsdoes not interfere with applying a magnetic force on the cantilever.

Magnetic force control according to the present invention isadvantageous in that magnetic forces can be used to modulate tip-sampleforces in atomic force microscopy. In practice, forces on the order of2-500 nN can be generated with an electromagnet and a small magnet onthe cantilever. The external magnetic force can be used to balancetip-sample interactions while measuring force versus distance curves orimaging, allowing mapping of forces in regions otherwise inaccessible.It can also be used to apply additional forces for applications such aselasticity mapping. The present invention has applicability for use influids such as gases or liquids.

The method and apparatus of magnetic force control of the presentinvention offers several advantages over known methods. The use ofmagnetic force typically offers advantages over use of electrostaticforces in atomic force microscopy. Also, as mentioned previously, theability to work in fluids is enhanced when using an atomic forcemicroscope in that images on many types of samples are improved. Sincefluids have dielectric constants, the change in capacitance as well ashydrodynamic forces between the plates may cause problems usingcapacitive techniques in fluid. Further, electric fields are affected bythe presence of most insulating or conducting material. This may createdifficulties in that the source of the field (the capacitor plate notattached to the cantilever) may be close to the cantilever, andtypically an intervening medium, such as a fluid, between the fieldsource and the cantilever will affect the field. On the other hand, mostmaterials are invisible to magnetic fields. As such, utilization of theapparatus and method for magnetic force control of the present inventionpermits the source of the magnetic field, such as the second magneticsource or electromagnet, to be large and positioned sufficiently remotefrom the cantilever without regard to many types of materials (forexample outside a glass fluid cell).

In contrast to the use of magnetic field gradients, advantages specificto using a magnetic torque are realized. First, the geometry of thetorque method is better suited for application to atomic forcemicroscopy than that of the field gradient method. In that magnetizationtends to lie along the geometrically thin direction of materials, suchas in the plane of thin films, it is generally easier to orient themagnetic moment along the cantilever by gluing a magnet flat orevaporating a thin magnetic film than it is to orient the momentperpendicular to the cantilever.

Also, larger forces may be generated using the torque method. In thefield gradient method, a gradient on the length scale of a permanentmagnet must be created to apply appreciable forces to the magnet.Typically, the size of the scanning probe, which generally is on theorder of one hundred microns, dictates the size of the magnet. As such,technical problems may result through the creation of huge fieldgradients. Conversely, it is relatively easier to create a field nearlyconstant on the length scale of the magnet by simply using anelectromagnet with dimensions large compared to the probe size.Considering the limits of the magnet size and on field gradients thatcan be currently achieved, the torque method promotes easierimplementation and promotes the generation of much larger forces.

For example, in experimentation using the method and apparatus formagnetic force control of the present invention, the electromagnetproduced a field of about 0.01 T (100 G) 2 mm from a smooth pole faceand a field gradient of 3 T/m (300 G/cm) the same distance from a sharppole. Since the cantilever magnet used was about 40 μm long, aneffective field gradient of 250 T/m was produced, about 80 times largerthan the real field gradient. Thus, larger forces could be applied formagnets with their magnetic moments parallel to the cantilever using thepresent invention.

Not only does the method and apparatus of the present invention using amagnetic torque give larger effective gradients over known fieldgradient methods, it is also much easier to implement. The highest fieldgradients are created very near a sharp magnet pole, but it is difficultto bring a sharp pole close to the cantilever from above withoutblocking the light path for sensing cantilever deflection. If the poleis underneath the sample there is a limit on sample thickness. On theother hand, it is easy to create a fairly constant field at thecantilever with the pole face spaced many millimeters from the sample.Also, if a thin magnetic film is used (either a magnetically hard orsoft film) it is easier to get the magnetization to lie in the plane ofthe film (cantilever). In experimentation using the present inventionwith evaporated iron, nickel and cobalt coatings, forces were generatedas high as 10 nN. However, the magnetic properties of the films werehighly variable. With better techniques, thin films could be a viableway to mass produce cantilevers suitable for magnetic force controlusing the present invention.

A further advantage of the present invention using a magnetic torque isthat it scales to smaller size more favorably. Since the moment of anobject scales roughly with the volume (assuming constant magnetization),if δ represents the length scale of the magnet, the force generated by agradient scales on the order of δ³, whereas the force generated by atorque scales on the order of δ². The result can be understoodqualitatively. As the moment becomes smaller, it becomes more difficultto create a gradient on its length scale, whereas maintaining a constantfield on its length scale becomes easier.

An advantage of the method and apparatus of the present invention isthat a weak cantilever may be used to provide good force resolutionwhile preventing snap-down until the gradient exceeds the "electronicspring constant" of the feedback loop.

Additional advantages of the method and apparatus of the presentinvention lie in the existing imaging modes of the atomic forcemicroscope. Resolution in the contact-mode depends on the sharpness andthe aspect ratio of the tip of the probe. As the sharpness and theaspect ratio are increased, the tip of the probe typically becomes morefragile. The method and apparatus utilizing magnetic force control ofthe present invention promotes preventing the snap-down when engagingdelicate tips for contact imaging.

The method and apparatus of the present invention also has applicabilityas to existing non-contact microscopes using interferometers. Inobtaining a linear response from an interferometer, it is known that thelength of the optical path must be set to a specified distance.Utilizing the present invention, rather than physically moving a fiberoptic cable or laser diode, magnetic force control of the presentinvention would be used to control the deflection of the cantileverportion of the probe.

Further, applicability of the present invention lies in the field ofnanolithography. With the method and apparatus utilizing the magneticforce control of the present invention, large forces can be applied tothe cantilever portion of the probe while in contact with a surface of asample to remove photoresist or physically pattern the sample, forexample. When such large force is discontinued, the cantilever portionof the probe is then used to image the pattern at a low force.

The present invention is also applicable to the use of the atomic forcemicroscope to dissect biological membranes. The method and apparatusutilizing magnetic force control of the present invention promotesachieving higher dissection forces without sacrificing the ability toimage at low forces which are usually required with biological samples.

A further advantage of the present invention relates to non-contactimaging of van de Waals forces at small separations, such as on theorder of less than 1 nm, by allowing non-contact resolution close tothat of the current contact modes and promotes eliminating destructivetracking and associated frictional forces.

As to an elasticity microscope, the present invention promotes theability to image at low force using a conventional contact mode todetermine topography and then image with a large imaging force appliedmagnetically, whereby differences between the two images providesinformation on local elasticity.

The ability to apply a large force directly to the end of the cantileveris useful in various applications. It is particularly useful inelasticity mapping because applying a loading force by bending thecantilever also causes the tip to slide relative to the sample,convoluting elasticity and friction information. Applying the forcemagnetically at the end of the cantilever using magnetic force controlof the present invention promotes circumventing this problem.

Additionally, the ability to apply forces directly to the cantilever(through any means) can be used not only to increase the effectivestiffness of the cantilever, but also to change other dynamicalproperties such as the effective resonant frequency or quality factor(Q). Magnetic force control according to the present invention in thisregard may be advantageous.

Obviously, additional modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method of magnetic force control for ascanning probe, comprising the steps of:providing a scanning probe,having a first magnetic moment along a first predetermined direction;and disposing a magnetic source external to said scanning probe to applya magnetic field to said scanning probe in a direction substantiallyperpendicular to said first direction so as to produce a torque actingon said scanning probe related to the amplitude of the applied magneticfield.
 2. The method of claim 1 in which said scanning probe is thecantilever of an atomic force microscope and in which said magneticfield modulates the end of the cantilever to the extent of, and inaccordance with, the imaging mode of the atomic force microscope.
 3. Amethod for changing the quality factor of the cantilever of an atomicforce microscope, comprising the steps of:providing a scanning probe,having a first magnetic moment along a first predetermined direction;disposing a magnetic source external to said cantilever to apply amagnetic field to said cantilever in a direction substantiallyperpendicular to said first direction so as to produce a torque actingon said cantilever related to the amplitude of the applied magneticfield; sensing the deflection of said cantilever; and applying saidmagnetic field with a value derived based on the sensed deflectionwhereby to change said quality factor of said cantilever.
 4. A magneticforce control apparatus for a scanning probe, comprising:a probe havinga magnetic moment along a first predetermined direction; and a magneticsource disposed external to said probe to apply a magnetic field to saidscanning probe in a second direction substantially perpendicular to saidfirst direction so as to produce a torque acting on said probe relatedto the amplitude of the applied magnetic field.
 5. A magnetic forcecontrol apparatus of claim 4 in which said scanning probe is thecantilever of an atomic force microscope, and further comprising:controlmeans causing said magnetic field to modulate the end of the cantileverto the extent of, and in accordance with, the imaging mode of the atomicforce microscope.
 6. A magnetic force control apparatus for an atomicforce microscope having a cantilever, enabling a change in the qualityfactor of said cantilever, comprising:said cantilever having a magneticmoment along a first predetermined direction; and a magnetic sourcedisposed external to said cantilever to apply a magnetic field to saidcantilever in a second direction substantially perpendicular to saidfirst direction so as to produce a torque acting on said probe relatedto the amplitude of the applied magnetic field; means for sensing thedeflection of said cantilever; and means applying said magnetic fieldwith a value based on the sensed deflection of said probe whereby toenable a change in the quality factor of said cantilever.
 7. A methodfor magnetizing a thin film of magnetizable material disposed on a forcesensing cantilever, comprising:placing the thin film of a magnetizablematerial on the force sensing cantilever; and magnetizing the thin filmso as to impose a magnetic moment therein in a direction that is alongthe length of the cantilever whereby when a magnetic field is applied tosaid cantilever substantially perpendicular to said direction, a torqueis produced acting on the cantilever related to the amplitude of theapplied magnetic field.
 8. The method of claim 7 in which the thin filmis magnetized whereby to impose said magnetic moment in a direction thatis along the length of the cantilever.
 9. A method for obtaining acantilever of a predetermined length bearing a thin film of magnetizedmaterial, comprising:depositing on said cantilever a thin film ofmagnetic material in a pattern along the cantilever length havingsubstantially greater length than width, whereby shape aniostropyconstrains the magnetic moment of the film to a direction that is alongthe length of the cantilever whereby when a magnetic field is applied tosaid cantilever substantially perpendicular to said direction, a torqueis produced acting on the cantilever related to the amplitude of theapplied magnetic field.